It is known in the engine connecting rod art to produce a two-piece engine connecting rod having a main body portion and a separable bearing cap portion from a one-piece unitary rod. A typical rod has an apertured large end portion from which the bearing cap is formed to allow the large end portion of the rod to be installed about the crankshaft's journal. A relatively old method for separating a bearing cap from a unitary rod is to actually saw the large end of the rod apart.
A more recent and preferred means of separating a bearing cap from a one-piece rod is by cracking or fracturing the rod. This method utilizes a longitudinally applied tension force of relatively great magnitude. The tension force causes the rod to crack and completely fracture. To control the cracking location and insure its position across the apertured large end portion of the rod, a stress riser is formed on the rod to accurately locate where the crack starts and to orient a desired fracture plane.
Stress risers are typically made in the form of a notch, groove, or scribe. Conventionally, stress riser notches have been broached or formed by a like mechanical cutting operation. Of course, as a broaching tool wears, the stresses riser notch becomes less distinct with rounded edges. This reduces its effectiveness and increases the tension force necessary to crack the rod. However, even using a sharp broaching tool, the V-shaped notch is too wide and shallow.
A recently discovered method to produce a desirable stress riser notch is to use a laser energy source to cut a narrow groove. This procedure is currently being practiced by Chrysler Corporation. The resultant laser formed stress riser is narrower and deeper than one formed by broaching. However, the laser operation is relatively complex and requires attention to achieve consistent results.
A preferred method of forming a stress riser in a rod is applicable only to connecting rods made from sintered powder metal such as steel particles. First, a green compact form of the rod is made by molding powder metal in a closed mold under significant pressure to mechanically interlock metal particles together. The green compact is sufficiently stable for handling but is not strong enough to be used as a rod. The mold also includes a configuration which forms a generally V-shaped notch in a side face of the green compact. Next, the green compact is heated in a furnace to about 2000 degrees F. and maintained at that temperature for a sufficient period of time to bond or sinter the metal particles into a strong, unitary structure. The resultant sintered preform or blank is much stronger than the green compact but still does not have sufficient strength and density for use as a connecting rod.
Then, the sintered blank is hot forged to increase density and strength. During hot forging, the thickness of the sintered blank is decreased and the spaced surfaces which form the V-shaped notch are folded inwardly toward one another to form a deep slit or crease with no substantial width.
Stress risers has been used to control rod cracking or fracture. Conventionally, stress risers have been located along the cylindrical surface of the aperture which extends through the large end portion of the rod. When a tension force is imposed on the rod, it is intended that the crack start evenly along the length of the stress riser and that the fracture progress straight across the rod. It has been found that when a stress riser is located in the above described cylindrical surface of the aperture, the maximum stress develops at one end of the stress riser adjacent the side thrust face and not evenly along the length of the stress riser. Resultantly, a small but significant percentage of such cracked rods have been found to include a small third piece of metal between the bearing cap portion and the main body of the rod. The formation of this third piece is unacceptable since it may interfere with accurate remating of the bearing cap with the main body. Such rods are scraped.
In accord with the above background of the manufacture of powdered metal connecting rods, the following are known: U.S. Pat. Nos. 32,117 to Weaver; 5,283,938 to Jones; 5,208,979 to Schmidt; 5,051,232 to Summers; 4,993,134 to Hoag et al.; 4,923,674 to Weber; 4,860,419 to Hekman; 4,838,936 to Akechi; 4,094,053 to Weaver; 4,049,429 to Donachie; 3,994,054 to Cuddon-Fletcher; 3,818,577 to Bailey et al.; 3,889,350 to Mocarski; 3,864,809 to Donachie; 2,648,578 to Stearns. The following foreign references are known: Japanese 58-141301; Japanese 55-15405; Japanese 5-277843;